In recent years, with the scaling down of a device, it becomes difficult to form a fine pattern and a fine pitch, and hence a double exposure process in which two photomasks are used in one layer is introduced to form the fine pattern. At present, in the double exposure process, a phase edge technique attracts the greatest attention. This technique is mainly used for gate formation, and in a region where a fine gate pattern is formed, a Levenson phase shift mask is used in addition to a photomask (binary mask or halftone phase shift mask) to form a normal pattern. The Levenson phase shift mask has the effect of, by arranging 0/π phase shifters on both sides of a pattern which becomes a gate electrode, inverting the amplitude of light in this region to raise a contrast. Consequently, it is possible to stably form even a pattern of 100 nm or less. Representatives of documents regarding this technique are U.S. Pat. No. 5,573,890 in which its principle is described and U.S. Pat. No. 5,858,580 in which a method of arranging shifters is described.
FIG. 7A to FIG. 7C are schematic plan views for explaining a case where gate patterns are formed by the phase edge technique.
A first photomask 111 is a first mask (binary mask or halftone phase shift mask) to form normal gate patterns as shown in FIG. 7A. This photomask 111 has gate patterns 101 to form gates, and the gate patterns 101 are exposed/transferred onto a photoresist on a substrate. Incidentally, for convenience, a portion of the gate, which is located on an active region 100 when the gate pattern 101 is transferred and the gate is formed, is called a gate electrode, and a portion thereof which is located on the other region (including an element isolation structure) is called a gate wiring.
A second photomask 112 is a Levenson phase shift mask (second mask) as shown in FIG. 7B. This photomask 112 has shifter patterns 102 corresponding to the gate patterns 101 of the photomask 111.
FIG. 7C shows an image obtained by overlapping these two photomasks 111 and 112. By aligning these two photomasks as shown and exposing the respective photomasks continuously (it makes no difference which photomask is exposed earlier), the gate patterns can be transferred to the photoresist on the substrate. It turns out that by using the Levenson phase shift mask as the photomask 112, the line width of the gate pattern only in a portion subjected to the double exposure process becomes very fine.
However, the phase edge technique of forming such fine patterns has a problem. The problem is a depth of focus. The Levenson phase shift mask realizes a high contrast as explained earlier, whereby a depth of focus sufficient to manufacture a device can be obtained. On the other hand, concerning the mask (binary mask/halftone phase shift mask) to form normal patterns, the patterns are formed in the vicinity of a resolution limit, whereby a sufficient depth of focus cannot be obtained in the gate patterns or the like which are not formed by the double exposure process. Moreover, also in gate electrode patterns formed by the double exposure process, it is similarly difficult to obtain a sufficient depth of focus when the photomask to form the normal patterns is exposed. Therefore, although the gate electrodes require highly uniform line widths in terms of device characteristics, dimension uniformity is degraded by variations in focus value when the mask to form the normal patterns is exposed.